Project Summary This project aims to develop and apply novel statistical approaches to address key causal questions in the of substance use, abuse and dependence. The development will focus on extending Mendelian Randomization methodology to new data types that test its key assumptions. These developments include: estimating and controlling for biases due to non- random mating; analyses of data from unrelated but genotyped individuals; extension to longitudinal data; multivariate network models; and multiple-group analyses to test for sex, age and other group differences. These new methods will be applied to unique longitudinal phenotype, genotype and methylation data on smokers and non- smokers from the Netherlands Twin Register, and data from the UK biobank. Precise identification of causal pathways will enable evidence- based prevention and treatment methods to be devised and implemented.